Thermocouple element



Patented Sept. 21, 1943 THERMOCOUPLE ELEMENT Leland R. van Wert, Philadelphia, Pa., assignor to Leeds & Northrup Company, Philadelphia, Pa., a corporation of Pennsylvania No Drawing. Application October 29, 1940, Serial No. 363,297

7 Claims.

My invention relates to thermocouple elements of alloys whose essential components are nickel and copper.

In accordance with my invention, to protect a cupro-nickel thermocouple element, from corrodents in a reducing atmosphere and from deterioration in an atmosphere alternately oxidizing and reducing, at least the surface of the element is comprised of the oxide or oxides of aluminum, having high heat of formation and resistant to reducing gases such as hydrogen, methane, carbon monoxide, or the like.

Preferably, aluminum, is alloyed with the copper and nickel not only to provide such metal for oxidation at the surface to procure a protective film or coating, but also to provide inter-granular segregates, principally oxide of aluminum, which in use of the element resist penetration along the grain boundaries by the reducing gases or corrodents; furthermore, compared with the base alloy of copper and nickel, the alloy formed by the addition thereto of aluminum has disproportionately increased resistance to corrosion so preventing or substantially precluding formation of fissures in the thermocouple element due to the joint effect of the reducing gases or corrodents and stress imposed upon the element either before or during its use.

My invention further resides in the thermocouple elements hereinafter described and claimed.

Premature structural failures of thermocouples in many cases are due to their having been disposed in atmospheres deficient in or lacking oxygen and containing corrodents such as sulphur dioxide, sulphur trioxide, hydrogen sulphide, carbon monoxide, and like gases usually present in furnace or other industrial atmospheres. The thermocouples under such conditions undergo substantial changes in composition and structure of the constantan oncopper-niokel element; the surface of the elementpractically entirely is composed of sulphide scale, mainly nickel sulphide; below the scale there is a zone of inter-granular corrosion of rather uniform depth from which there extend fissures where inter-granular corrosion has proceeded far more rapidly. The lastnamed phase of the corrosive attack, formation of fissures, is the immediate cause of structural failure, by breakage, of the thermocouple element or conductor, with consequent termination of the useful life of the thermocouple.

Other structural failures occur under conditions of exposure to furnace or other industrial atmospheres alternately oxidizing and reducing. The

thermocouple elements which fall under these conditions also suffer changes in composition and wastage, causing substantial changes in physical dimensions and breakage.

During each oxidizing period, the surface of the element appears to oxidize and the oxide so formed is removed in the succeeding reducing period, so decreasing the size of the thermocouple element until eventually it breaks under the stress of its own weight or the stress imposed by its mounting which in some cases is similar to that shown in U. S. Letters Patent 2,151,648- to Baker.

The decrease in dimensions of the thermocouple element under aforesaid conditions is also accompanied by change in its composition, especially at or near its surface. It is believed because of the difference between the heats of formation of the oxides of nickel and copper, there is selective oxidation during each of the oxidizing phases of use, the formation of nickel oxide proceeding at more rapid rate than the formation of copper oxide; the heat of formation of nickel oxide not being sufliciently high to render its oxide stable in the presence of active reducing gases such as hydrogen, the alloy at and near the surface of the thermocouple element is converted, after a series of these oxidizing-reducing cycles, from' cupro-nickel alloy to practically pure copper.

In consequence, the temperature-voltage characteristic of the thermocouple further and further departs from the characteristic upon which calibration of the associated indicating, recording or controlling apparatus is based. Consequently, even before mechanical failure of the thermocouple element enforces replacement of the thermocouple, its useful life as a voltage-producing device for use with precalibrated measuring apparatus has terminated.

The life of thermocouples to be used under any of aforesaid conditions is substantially prolonged, in most cases at least several hundred percent, by providing that at least the surface of the thermocouple element shall comprise aluminum oxide whose heat of formation (about 400 kilogram calories) is substantially higher than the heats of formation of the oxides of copper and nickel (about 35' to 58 kilogram calories) and which oxide is stable under conditions of exposure at elevated temperatures to reducing gases such as hydrogen, carbon monoxide, methane and the cally inactive with respect to its ambient atmosphere.

Although the protective coating of oxide can be formed in various other ways, I prefer, for reasons hereinafter appearing, to add to the basic alloy of copper and nickel or to the initial melt of the copper and nickel a suitable percentage of aluminum to form a ternary alloy, of which it is characteristic there are inter-granular segregates, principally oxide or oxides of the aluminum, which are resistant to the reducing or corrodent gases and so prevent their penetration-into the thermocouple element along the grain boundaries. There is thus prevented the fonnation of intergranular corrosion fissures even though the surface of the element be not protected, or not sufflciently completely so, by a1r-oxide coating.

The aforesaid surface coating of oxide may be formed on the element of said ternary alloy either by heating or annealing the element, prior to use, in suitable atmosphere, for example in commercial hydrogen, or in mixtures of hydrogen and nitrogen of commercial purity, or by heating or annealing the element as an incident of its actual use in an atmosphere which is essentially neutral or reducing and containing at least a trace of one or more oxidizers, such as oxygen, water vapor, carbon dioxide, or the like.

The oxide film is tenacious, has little tendency to scale, is substantially impervious to corrodents and reducing gases, and is self-healing if the atmosphere in which the thermocouple is used contains as much as a trace of an oxidizer such as oxygen, carbon dioxide, water vapor, or the like.

As compared with the binary alloy of copper and nickel, usually copper 40% to 60%, nickel 60% to 40%, the ternary alloy produced as aforesaid has a much higher tensile strength with resultant disproportionately enhanced resistance 01' the thermocouple element to,stress-corrosion, an advantage especially significant when the thermocouple is used at high temperatures, for example in the range from 700 to 1400" F. In consequence, even though in course of time there is penetration of the element by corrodents, the stress-corrosion, responsible for formation of fissures, proceeds at greatly reduced rate with correspondingly longer useful life of the thermocouple.

In general, the percentage by weight of aluminum two or more of them, added to the basic alloy of copper and nickel lies within the range of from about one-quarter percent as a lower limit to about ten percent as an upper limit; at least sufiicient of the aluminum should be added to insure formation of the inter-granular oxides and to attain at least the desired stress-corrosion resistance. The upper limit is based upon other considerations such as difiiculty of working or machining the thermocouple element or the stock from which formed, and the desired voltage-temperature characteristic of the thermocouple.

. Throughout at least the range of temperatures measured by thermocouples, the solubility in the basic copper-nickel alloy of aluminum, unlike that of silicon and beryllium, for example, is invariable, and so affords that constancy of composition ensuring at most inappreciable departure from aforesaid desired voltage-temperature characteristic.

Instead of mixing aluminum with the coppernickel melt, a cupro-nickel thermocouple element may be exposed in a reducing atmosphere at suitably elevated temperature to aluminum powder. The resulting aluminum coating is slowly converted to aluminum oxide by exposure to air at room temperature, or more rapidly by heating, before or during use, in an atmosphere including at least a trace 01. one or more oxidizers, and'protects the thermocouple element, as previously herein explained. Under condition or exposure to high temperature, before or during use of the thermocouple, the aluminum oxide diifuses into the element with the same end result procured by initially alloying the aluminum with the copper and nickel.

Compared with the base alloy of nickel and copper, the ternary alloy resultingfrom addition of aluminum exhibits against platinum a somewhat lower average thermoelectric potential, the magnitude of the decrease in potential being greater the greater the percentage of aluminum added; but nevertheless a desired new or old voltage-temperature characteristic of the thermocouple may be obtained by proper selection of the other element of the thermocouple resorting, if necessary, to variation of the percentage composition of certain components of that element which, when a ferrous alloy, may have its content of non-ferrous materials varied in accordance with the disclosure of copending application Serial #311,258, filed December 28, 1939, by Donald 1. Finch.

From Table A below comparison may be made between the thermoelectric power against pure platinum of the usual copper-nickel or constantan thermocouple element and the thermoelectric powers, in microvolts against pure platinum, of alloys comprising a base of the same composition as the copper-nickel or constantan with addition of specified amounts of aluminum.

TABLE A Temperatures vs. microvolts Base Buoy Base alloy plus aluminum Temp. 0. Cu 54.76%

Microvolts Microwlts Mcrovolta Microvalla Table B below indicates the increased tensile strength of the element procured by addition to constantan of specified amounts of aluminum; it should be particularly noted that at high temperatures, at which stresscorrosion resistance is particularly necessary, the tensile strengths of Tm: B

[Tensile strengths in pounds pull to break wire, No. 8.

l3. 5n 8. gauge] Room tempera- 750 F 1350 F.

ture

Gonstantan (Cu 55%, Ni 45%).... 749 593. 5 181. 9 Oonstantan .72% Al (1.13 Mn).- 945 811.0 232 Constantan 1.22 Al 914.5 766. 5 263.5 Constantan+262 Al 1, 060. 889 250 The foregoing ternary alloys of Tables A and 10% by weight, said aluminum providing oxidized aluminum at the suriace oi the theme-couple element.

4. A thermocouple element consisting of alumi- 5 num alloyed with copper and nickel, said alumi- B having particular potential-temperature characteristics and tensile strengths are given by way 0! example without limitation upon the broader aspects 0! my invention as previously herein set forth and as defined by the claims hereof.

What I claim is:

1. A thermo-couple element consisting oi an alloy of copper and nickel alloyed with about 0.25% to 10% by weight of aluminum.

2. A thermo-couple element consisting of an alloy consisting substantially of copper about 40% to 60% by weight, nickel about 60% to 40%. and aluminum about 0.25% to 10%.

3. A thermo-couple element consisting of an alloy of copper, nickel and aluminum, the lastnamed present in proportion of about 0.25% to num being present in proportion oi about 0.25% to 10% by weight and providing in said element inter-granular segregates of oxidized aluminum.

5. A thermo-eouple element consisting of an alloyhaving the proportions by weight of copper about to nickel about to 40%. and

aluminum about 0.25% to 10%, said aluminum providing oxidized aluminum at the surface oi the thermo-couple element.

6. A thermo-couple element consisting of an al- 

